In a key-shift transmission, a speed change is achieved by shifting a shift collar on a shaft so as to displace gear-engaging lugs of shift keys to a position where the lugs are projected radially outwardly from elongated grooves in the outer surface of the shaft into recesses of one of freely rotatable change gears so as to couple such one gear to the shaft. For biasing a shift key so as to project a gear-engaging lug thereof outwardly, there have been used two ways. One of the ways employs a self-biasing key made from a resilient material into an appropriated configuration, as shown in, for example, U.S. Pat. No. 3,812,735. The other way employs a separate leaf spring which is affixed to the inner surface of a shift key, as shown in, for example, U.S. Pat. Nos. 2,395,433 and 3,028,763. Of the latter two U.S. patents U.S. Pat. No. 2,395,433 discloses a resiliently deformable shift key to which a leaf spring is associated, whereas U.S. Pat. No. 3,028,768 discloses a shift key having an end portion pivotally connected to a base half of the key and a leaf spring affixed to the rotatable end portion.
The former way employing a self-biasing key has a drawback that repeated uses of such key will cause a considerable fatigue of the key resulting in a lack of biasing force and, therefore, gear-coupling force which in turn will result in an uncertainty of speed change transmission. The latter way employing a separate leaf spring will reduce the durability of a shift key, because the leaf spring is arranged within an elongated axial groove in the outer surface of a transmission shaft at the inside of the shift key in a fashion such that the spring is slidably received at a portion thereof by the bottom surface of the elongated groove and because for permitting such arrangement of the leaf spring at least a portion of the shift key is considerably reduced in thickness in the radial direction of the shaft and is thus reduced in strength.
JP, A(J) (Japanese Utility Model Publication under Art. 13bis of the Japanese Utility Law) No. 60-65453 discloses a key-shift transmission which eliminates the drawbacks set forth above. In this transmission, shift keys are pivotally supported at base end portions thereof by a shift collar within which compression springs acting upon the base ends of the shift keys are accomodated for biasing the keys. The compression springs seperate from the shift keys will not cause a fatique of the keys. Such springs also will not reduce strength of the shift keys, because they are arranged outside elongated axial grooves in a transmission shaft so that no reduction of thickness of the shift keys is required.
However, the structure disclosed in JP, A (U) No. 60-65453 will cause a different problem.
In the key-shift transmission shown in JP, A (U) No. 60-65453, shift keys are formed near their base ends with integral projections which extend radially outwardly of a transmission shaft having keyways or elongated axial grooves. Such projections are received in slits formed to an annular key-supporting portion extending axially from the shift collar and are pivotally connected to the supporting portion through pins for providing a pivotal connection of shift keys to the shift collar. The shift collar further has a plurality of tubular spring-accomodating portions which extend radially outwardly from an axial mid portion between the key-supporting portion and a base end shifter-engaging portion. Compression coil springs for biasing shift keys are accomodated within such tubular accomodating portion so as to act upon the base ends of shift keys through balls. It is thus seen that the shift collar has an enlarged axial length due to the additionally provided key-supporting portion and spring-accomodating portions. The plurality of spring-accomodating portions will enlarge substantially the outer diameter of the shift collar.
The enlarged axial length of the shift collar will enlarge the length of transmission shaft mounting such collar to cause an enlargement of the width of a transmission casing. The enlarged outer diameter of the shift collar which rotates in unison with the transmission shaft will provide limitations to the arrangement of transmission members so that compactness of the transmission is impeded.
Accordingly, a primary object of the present invention is to provide a novel key-shift transmission which secures a compactness of the transmission while eliminating the drawbacks referred to before.